A conventional imaging device has a lens that focuses incident light, an imaging device that converts the focused optical image to an electrical signal, and an image signal processing means that processes the electrical signal obtained from the imaging device to obtain a prescribed image signal.
To resolve different colors, when the imaging device is a single-element sensor having a single CCD (Charge Coupled Device) or CMOS (Complementary Metal Oxide Semiconductor) sensor, these being the types of sensors usually employed as imaging elements, a filter of a different color is required for each pixel.
To obtain red (R), green (G), and blue (B) color signals, red, green, and blue primary color filters that pass optical bands corresponding to red, green, and blue may be used, or magenta (Mg), cyan (Cy), yellow (Ye), and green (G) complementary filters may be used. Filters of each of these colors are designed with dyes or pigments having spectral transmission characteristics that pass light of the desired color, but they also have a certain transmittance in the near infrared region. The principal component of the photoelectric conversion section of the imaging element is a semiconductor such as silicon (Si), so the photoelectric conversion section is sensitive to the part of the near infrared region with comparatively long wavelengths. The signal obtained from an imaging element equipped with color filters therefore responds to light rays in the near infrared region.
The chromatic curves that express human sensitivity to color and the brightness curve that expresses human sensitivity to brightness are located in the so-called visible spectral region from 380 nm to 780 nm in which sensations can be perceived, but there is barely any sensitivity at wavelengths longer than 700 nm. To match the color reproducibility of the imaging device to the human chromatic curves, it has been necessary to make a visual sensitivity correction by placing an infrared cut filter (hereinafter, IRCF) in front of the imaging element to block near infrared rays.
When sensitivity is more important than color reproducibility, however, as in a surveillance camera, it is better not to provide an IRCF and to allow the imaging element to receive near infrared light, so that the near infrared part of the spectrum can be used.
Conventional imaging devices address the above situation by incorporating an optical filter means for making a visual sensitivity correction in coordination with the diaphragm means that adjusts the amount of incident light, to obtain good color reproducibility by using the filter means to make a visual sensitivity correction when the diaphragm is stopped down. Below a certain level of illumination, however, the optical filter is removed from the optical path by an operation coordinated with the diaphragm so that high-sensitivity imaging is performed using the infrared light. An IRCF is used as the optical filter means for the visual sensitivity correction. When the visual sensitivity correction is performed, light of wavelengths equal to or greater than a certain wavelength (more specifically, equal to or greater than 650 nm) is attenuated (see, for example, Patent Document 1).
Another conventional imaging device uses an infrared cut filter to attenuate infrared light during high-sensitivity imaging, and uses an infrared cut filter and a band attenuation filter that attenuates yellow-green during imaging that combines high sensitivity with good color reproducibility. The image signal is obtained by switching filters to adjust the mixing ratio of the red, green, and blue signals according to the imaging purpose (see, for example, Patent Document 2).
Yet another conventional imaging device has means for output of a prescribed color image signal in imaging with at least a prescribed amount of incident light or with a light source that produces mainly visible light, and for output of a monochrome image signal in imaging with less than the prescribed amount of incident light or with a light source that produces mainly infrared light. When the prescribed color image signal is output, the white balance is corrected by using a prescribed mixing ratio of the red, green, and blue signals (see, for example, Patent Document 3).    Patent Document 1: Japanese Patent Application Publication No. 2001-36897 (pp. 3-6, FIGS. 1-5)    Patent Document 2: Japanese Patent Application Publication No. 2003-134522 (pp. 3-5, FIG. 1)    Patent Document 3: Japanese Patent Application Publication No. 2003-264843 (pp. 5-6, FIG. 1)